Intravascular probes which include ultrasound imaging crystal arrays have been proposed in the past. It is known, for example, to mount a piezoelectric crystal element (conventionally termed a "transducer") on or within a catheter of the type which can be inserted into a blood vessel. Once the probe has been inserted into a blood vessel, the transducer is electro-mechanically excited (as by the application of an electrical signal) to cause emission of ultrasonic energy into the surrounding tissue. While much of the emitted energy is absorbed by the surrounding tissue, a sufficient amount of energy is reflected back toward the transducer to permit imaging (with reflection occurring principally at interfaces between different types of material, e.g., the interface between blood and the vascular wall, the interface between blood and lesions adhered to the vascular wall, etcetera).
The transducer, in turn, produces weak electrical signals in response to electro mechanical excitation by the returning reflected ("echo") ultrasonic energy. These weak electrical signals can be used to determine the geometry and/or other characteristics of the blood vessel, for example, to determine whether or not the blood vessel contains lesions or other abnormalities. These determinations are usually termed "imaging" since suitable video and/or other signal monitoring equipment are employed to convert the weak electrical signals produced by the transducer into human-readable form. Information gained from such imaging thus may assist the physician in a vascular treatment in real time or in diagnosing a patient's particular ailment or disease so that suitable therapy can be prescribed.
One problem that has plagued conventional ultrasound imaging probes in the past is that an inherent "dead space" usually exists in the immediate vicinity of the transducer. That is, since considerable mechanical excitation (i.e., vibration) of the transducer occurs when ultrasonic energy waves are generated and emitted, it takes some time thereafter for the ringing crystal structure to quit vibrating sufficiently to permit detection of the much weaker echo reflections being returned to the transducer. Once this "dead" time is elapsed, the transducer can begin responding to received echo waves. The time it takes for the transducer to cease its strong electro mechanical transmit vibration (i.e., so that it can then begin sensible electro mechanical vibration in response to weaker echo waves) is sometimes termed the transducer "ring down" time. As can be appreciated, at any given transducer frequency of operation, more or less ring down time will inherently be present so that a region surrounding the transducer is effectively masked--that is to say, the ring down time creates a "dead space" in the immediate vicinity of the transducer where no imaging is possible.
The dimensional extent of such dead space is dependant upon many variables, including the frequency of operation of the transducer. Suffice it to say here that although transducer dead space can be tolerated when relatively large intravascular cavities are imaged (i.e., relative to the size of the imaging probe), significant problems are encountered when small intravascular cavities, such as small diameter blood vessels, etcetera, are to be imaged. And, in any event, transducer dead space mitigates against miniaturization since even the smallest diameter imaging probe is only capable of imaging intravascular cavities outside of its surrounding dead space, thereby providing for an effective imaging area which is usually only significantly greater than the probe's diameter.
Another problem which mitigates against imaging probe miniaturization is that the transducer must be "tuned" to the electrical cabling which supplies driving signals to, and returns weaker echo electrical signals from, the transducer. That is, since the transducer, at its frequency of operation, exhibits a net capacitive reactance, inductive reactance should be provided so as to efficiently couple the transmit/receive signals to the transducer (e.g., so as to maximize signal-to-noise ratios).
The present invention, however, provides a miniaturized ultrasonic imaging probe which not only exhibits essentially zero "dead space" (i.e., intravascular imaging can be accomplished in blood vessels having, or capable of being dilated to, substantially the same diameter as the probe itself) but also provides for internal (i.e., as part of the probe per se) inductive reactance. And, the fact that internal inductive reactance may now be provided enhances the ability to use weak electrical echo signals for purposes of diagnostic imaging.
These novel features of the invention are achieved by equally novel structure associated with an ultrasonic imaging probe of the type including a probe guide assembly, a transducer connected to the distal end of the probe guide assembly, and electrical cabling housed within the probe guide assembly and operatively connected to the transducer for transmitting electrical power to, and receiving electrical signals from, the transducer. The transducer is mounted within a proximal end portion of a generally cylindrical holder (itself being attached to the distal end of the probe guide assembly) which defines an elongate, open trough. Thus, the transducer is also mounted near the proximal end of the trough.
An ultrasound reflector (e.g., polished stainless steel) is mounted at the distal end of the holder (i.e., at the distal end of the defined trough) in axially spaced relation to the transducer for directing ultrasonic energy waves between a first path (which is substantially parallel to the elongate axis of the probe) to a second path (which is substantially perpendicular to the elongate axis of the probe). Thus, ultrasonic energy waves emitted from the transducer along the probe's elongate axis will be redirected radially of the probe by means of the reflector towards the surrounding tissue being imaged. Echo waves which radially return from the surrounding tissue will likewise be redirected by means of the reflector axially of the probe towards the transducer. The distance that the reflector is axially spaced from the transducer is selected such that the "dead space" lies substantially therebetween. Thus, since the dead space for any given transducer will lie axially between the transducer and the reflector, intravascular imaging immediately radially adjacent the external periphery of the probe is possible. That is, as long as the probe is of a size which is capable of being moved within intravascular tissue or organs, then imaging of such tissue or organs is possible.
The probe of the invention is also provided with an inductor coil as the inductive reactance, the inductor coil being coaxially housed within the probe guide assembly and positioned closely adjacent the transducer. Preferably, the electrical cabling of the invention is a standard coaxial cable having an inner conductor (with an associated insulating layer) and an annular outer conductor (with an associated insulating layer). According to this invention, a distal end segment of the outer conductor (and its associated insulating layer) is removed. The inductor coil is then coaxially positioned over the inner conductor (and its associated insulating layer) in the space previously occupied by the removed outer conductor segment. The proximal end of the inductor coil is thus connected to the distal end of the remaining outer conductor while the distal end of the inductor coil is connected electrically to the transducer. The inner conductor, on the other hand, is connected electrically to the transducer at a location different from the connection of the inductor coil (preferably at a location on the front face of the transducer). In such a manner, the transducer and inductor coil are connected closely adjacent, and in series relationship, to one another.
These as well as other objects and advantages of the invention will become apparent after careful consideration is given to the following detailed description of the presently preferred exemplary embodiments.